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This invention relates generally to human hearing, and more specifically to the design and positioning of an electrode array for a cochlear prosthesis.
Human deafness results from numerous factors including trauma, ear infections, congenital factors, ototoxic effects of some antibiotics, and from diseases such as meningitis. Sensorineural damage (damage to the hair cells in the cochlea) is the largest single form of hearing loss. In a healthy ear these hair cells convert acoustic signals in the inner ear to electrical signals that can be interpreted by the brain as sound. It is estimated that over 7% of the U.S. population is affected by sensorineural deafness, and one in a thousand infants is born totally deaf. Extrapolating these percentage figures, it is estimated that there are 30 million people in the world who are profoundly deaf.
Considerable research over the past several decades has been directed towards developing a means to bypass the non-functioning hair cells in the inner ear (or cochlea) by using electrodes to directly stimulate auditory afferent neurons within the cochlea. This so called cochlear implant technology has progressed from early methods of attaching one or more single wire electrodes onto the promontory or the bony shell of the cochlea, to drilling directly into the cochlea, and inserting electrodes into the scalae therein. Electrodes used in modern cochlear prostheses generally use a longitudinal bipolar (or monopolar) electrode configuration where small platinum/iridium balls or circular platinum rings connected internally by thin wires, with the electrodes and wires held together in a smooth elongated silicone carrier, are surgically implanted into the scala tympani (one of the canals within the cochlea), via a hole made in the mastoid bone behind the ear. Entry into the scala tympani is generally via the round window membrane. The electrodes are electrically connected to an electronics package anchored in a cavity made in the mastoid bone. Information is sent to this internal (subcutaneous) electronics package, via RF transmission across the skin barrier, from an external body-mounted electronics package that houses the speech processor, control electronics and power supply.
Such cochlear prostheses are commercially available from a number of companies worldwide, for example, from Cochlear Limited, Sydney, Australia; Advanced Bionics Corporation, Sylmar, Calif., U.S.A.; Med-El Medical Electronics, Innsbruck, Austria; PHILIPS-Antwerp Bionic Systems N.V./S.A., Edegem, Belgium; and MXM Medical Technologies, Vallauris, France.
The surgery time and surgical complexity of implanting these commercial cochlear prostheses is significant, especially for very young and for old persons. The implant procedure usually involves exposure of the mastoid cortex of the implanted ear via elevation of a postauricular skin flap, generally requiring 2.5-4 hours with the patient totally anesthetized, and with the inherent medical risks of total anesthetic. The cost of currently available cochlear prostheses is high, limiting the availability of this technology mostly to the wealthy industrialized countries. A comprehensive introduction to the development of cochlear implants is given in, for example, xe2x80x9cCochlear Prostheses,xe2x80x9d edited by G. M. Clark, Y. C. Tong and J. F. Patrick, distributed in the U.S.A. by Churchill Livingstone Inc., New York, N.Y. 1990 (ISBN 0-443-03582-2), and in xe2x80x9cThe Cochlear Implant,xe2x80x9d (ISSN 0030- 6665) by T. J. Balkany, editor of The Otolaryngologic Clinics of North America, Vol. 19, No. May, 2, 1986. Additionally, some of the early cochlear implant work is described in U.S. Pat. Nos. 4,357,497; 4,419,995 and 4,532,930.
In spite of the surgical risks, complexity and device costs, currently available cochlear prostheses do provide a major improvement over the alternativexe2x80x94total silence. However, there are still great differences in hearing percepts amongst implanted patients. Some patients after implantation are able to use the telephone, while others can only perceive environmental sounds. Also, there is the great inconvenience, and social stigma, especially for children, in needing to wear an external head-mounted device, connected to a body-worn (or a recently available ear-mounted) electronics package. A totally implanted cochlear prosthesis does not yet exist.
Researchers have tried for many years to ascertain the reasons for the variable hearing results obtained by cochlear implant patients. The consensus of scientific opinion is that the location of the electrodes in the cochlea, age of implantation, time of implantation since deafness, duration of implant use, knowledge of language prior to implantation, duration and intensity of rehabilitation, and patient ability and desire to learn are key factors in determining speech understanding by the implantee. Other factors include the number of functional peripheral neural processes in the basilar membrane and cochlea spiral lamina and/or surviving spiral ganglion cells in the modiolus, the type of speech coding strategy used, and the extent of trauma from surgery during implantation.
The electrode array design parameters are important. For example, the number of electrodes and the spacing between electrodes, the position of electrodes in the scala tympani (or scala vestibuli) with respect to the stimulatable neural sites, and the orientation of the electric field generated between electrode pairs are all factors affecting patient speech percepts.
In spite of all of the potential factors contributing to hearing results, it is clear that the functionality of a cochlear prosthesis will always be limited by the intrinsic design and positioning of the electrode array within a scala. Interestingly, the simple tapered longitudinal bipolar and monopolar electrode arrays using small platinum/iridium balls or circular rings that are now widely used commercially were developed over a decade ago, and were largely based on the relative ease of fabrication and the practicality of surgical insertability, rather than on critical design parameters necessary to achieve optimum neural stimulation.
The use of the simple longitudinal multi ring electrode design, where the rings are held in place with a silicone carrier, has the advantage of simplicity and does not require rotational orientation within the scala. This design is now commonly used by most commercial manufacturers of cochlear implants. However, the longitudinal electrode configuration not only creates an unwanted bimodal distribution of exited nerve fibers (Frijns, J. H. M. et.al., Hearing Research, 95 (1996), 33-48) but only a small segment of the annulus-shaped electric field generated between electrode ring pairs stimulates nerve fibers, thereby wasting a large amount of electrical energy. Frijns, et. al. also suggest that the efficacy of neural stimulation is enhanced when the electric field lines are parallel to the nerve cells rather than perpendicular as is dictated by the longitudinal configuration. Additionally, electrical cross-talk between adjacent ring electrodes becomes an increasing problem as more electrodes are used to obtain better spatial selectivity along the length of the scala.
The small diameter discrete electrode (ie. platinum/iridium ball) type designs have the distinct problem of rotationally twisting during insertion of the array, resulting in some or all of the electrodes facing away from the stimulatable neural sites. Accordingly, designs have been proposed whereby the array preferentially bends in one plane, while maintaining rigidity in the other plane. For example, Charvin, in U.S. Pat. No. 5,123,422, teaches the use of internal hinges or slits, where such hinges or slits are oriented to give flexibility in only one plane, and can be inserted in the scala tympani without curling, thus orienting the electrode sites xe2x80x9cto obtain good stimulation of the nerve cellsxe2x80x9d. Hansen, in U.S. Pat. No. 4,261,372, uses xe2x80x9cVxe2x80x9d shaped notches along one side of the array to permit the array to assume the required curved shape within the scala, and to obtain greater insertion depth of the electrodes by first inserting one part of the electrode into the first turn of the scala tympani and then inserting the other part into the second turn of the scala tympani. Jarvik, et. al., in U.S. Pat. No. 4,832,051, describe an electrode device where xe2x80x9cthe elements are resiliently attached together so that the stack of elements is stiff in compression along the common axis and is flexible in tension.xe2x80x9d
None of the above patents, however, addresses the additional critical issue of positioning the electrodes in close proximity to the peripheral processes in the spiral lamina or the spiral ganglion cells in the modiolus. To achieve such proximity requires the electrodes to be positioned snug against the modiolar wall of the scala. To address this proximity requirement, a myriad of electrode array designs have been suggested. For example, Hansen, et. al., in U.S. Pat. No. 4,284,085, use an electrode with two conditions of curvature, where one curvature is temporary, and the other is permanent. The temporary curvature allows insertion with minimum surgical trauma, and by means of a detachable connection, the array assumes a position to obtain an optimum final position relative to the acoustic nerves over the entire electrode length. Michelson, in U.S. Pat. No. 4,400,590, teaches a multichannel electrode array comprised of discrete bipolar radially positioned electrode pairs on one side of the array, where such array is positioned to xe2x80x9cbe located adjacent predetermined auditory nerve endings in the basilar membranexe2x80x9d, a location that seems, in hindsight, not an optimum location for neural stimulation as the nerve fibers in this locale are unmyelinated and thus difficult to stimulate. A design by Jacobs, in U.S. Pat. No. 5,061,282, similarly tries to stimulate neurons connected to auditory nerves in the basilar membrane using a plurality of transducer elements disposed along the length of the cochlea adjacent to the basilar membrane. Byers, et. al., in U.S. Pat. No. 4,819,647, describe a multichannel array where the electrode conductors have elongated cross-sections which are aligned to allow the array to readily flex in the plane defined by the array spiral, limiting flexing in the vertical direction, with the overall array having spiral shape corresponding generally to the shape of the scala tympani. The micro ball-shaped electrodes are positioned at right angles with respect to each other, and displaced longitudinally from one another, where one electrode is positioned facing the basilar membrane and the other facing the modiolus. Stypulkowski, in U.S. Pat. Nos. 4,961,434; 5,00,194 and 5,037,497, describes a number of designs for a cylindrical or tapered cylinder flexible array with various flush and recessed electrode surfaces in a radial bipolar configuration, with details of fabrication methods for such devices. However, these designs do not allow for either rotational positioning within the scala tympani to orient the electrodes nor do they allow for overall positioning of the array near the modiolus or spiral lamina. Kuzma, in U.S. Pat. Nos. 5,545,219 and 5,645,585, illustrates a flexible electrode carrier connected to a flexible positioning member used to force the electrode carrier into a close modiolus-hugging arrangement and for disposing the electrode surfaces towards the spiral ganglion cells in the modiolus. In a further patent (U.S. Pat. No. 5,578,084) Kuzma, et. al., disclose an electrode array design whereby one part of the array material slowly absorbs water from the surrounding cochlear perilymph fluid, thereby expanding the array to engage the electrodes to the modiolar (or inner) wall of the canal. In another version of this concept, Kuzma et. al., in U.S. Pat. No. 5,653,742, further disclose an electrode carrier that is either pre-formed and held with a bioresorbable stiffening element which dissolves in the cochlear fluid after implantation, or contains biasing fms held folded against the carrier which can flex outward upon dissolution of the holding sheath, such that the fins or pre-curved shape of the array act to position the array towards the modiolar wall. Spelman et. al. in U.S. Pat. No. 5,800,500, teach an electrode design comprised of a plurality of insulated wires wrapped around a tube containing a memory shape wire, where selected areas of wire insulation are removed with a laser, both longitudinally along the length of the array and laterally along the array circumference. The memory shape wire (or polymer) acts to position the array towards the modiolar wall upon reaching body temperature.
The electrode array devices that have been disclosed suffer from a variety of limitations which predispose them to meeting some criteria while not meeting others. The electrode array criteria that appear to provide for optimum performance are:
(a) electrode proximity to spiral ganglion cells in the modiolus and the peripheral processes in the spiral lamina,
(b) optimal orientation of the electric fields generated by paired electrodes,
(c) a high density of electrodes (electrodes per unit length) without significant signal cross-talk between electrode pairs and
(d) surgical insertability of the electrode array into the entire length of the scala, generally the scala tympani, especially to the apical end.
Since nerve fibers stimulated with an electric field parallel to said nerve fibers have a lower stimulation threshold, an optimum electrode orientation would be radially bipolar. However, it is essential that the electrodes be positioned as close as possible to the stimulatable nerve fibers so as to minimize threshold currents. Anatomically, there appear to be two distinct locations, one near the modiolus to stimulate the spiral ganglion cells, and one near the spiral lamina to stimulate the peripheral processes therein. However, these two stimulation sites are orthogonal one to the other, thus requiring an orthogonal electrode design to accommodate both stimulatable sites. Interestingly, stimulation near the basilar membrane would require high currents, since the fibers therein are unmyelinated, with fiber myelination starting near the habenula perforate. Since the electrodes used in the invention can be positioned close to the spiral lamina, it is possible to achieve high spatial selectivity and high electrode density, although such stimulation must be done shortly after onset of deafness since the peripheral processes in the spiral lamina tend to atrophy relatively quickly if not stimulated. Stimulation of the spiral ganglion cells in the modiolus requires the electric field to extend further into the nerve bundle, although this increases the probability of ectopic stimulation (or stimulation of fibers in scalae turns not containing the stimulating electrode). This aspect may be somewhat alleviated by using an electrode array which hugs the modiolar wall in each of the scala tympani and scala vestibule.
Surgical insertability of the array is critical. However, the array stiffness requirements are contradictory. Said array must be sufficiently stiff to be insertable, especially to traverse the sharp bends in the scala tympani between the round window and the first turn, yet be highly flexible so as to avoid damaging the delicate structures within the cochlea. Accordingly, most conventional devices use a soft, flexible medical grade silicone carrier, which is difficult to fully insert, and some cochlear implant manufacturers provide specialized insertion tools to assist in the insertion of their electrode arrays. The invention addresses this issue through the use of a simple microformed polyfluorocarbon carrier attached to lithographically created electrodes and conductors (embedded in a polyfluorocarbon film), where such an array can be conveniently and safely inserted into the scala tympani by a surgeon.
Ease of fabrication of the electrode array is also essential. Most conventional arrays rely on manual fabrication techniques that lack process control. The lithographic technique used in this invention facilitates low cost non-manual fabrication which allows for greater process control and consequently greater reliability of the frnished product. This technique also allows for higher conductor wire density than can be achieved with conventional fabrication techniques. The object of this invention is to provide an implant that overcomes the limitations of the prior art, as well as to provide an improved method of surgical positioning of such a device.
The invention comprises an electrode array containing three main components, namely:
(a) a bio-inert film holding conductor lines and at least two radial bipolar oriented electrodes, and
(b) a bio-inert carrier with an opening or hole substantially through the center of the longitudinal axis of the carrier, and, in a further embodiment, with at least one partially circumferential notch (or xe2x80x9cVxe2x80x9d groove) disposed along the longitudinal axis of the carrier, and
(c) a beading designed to slide smoothly within the longitudinal opening or hole in the carrier
The film, carrier and beading are preferentially fabricated from a flexible biocompatible material, such as the polyfluorocarbon FEP, where said carrier has, on opposing sides, one rounded surface and one substantially flat surface, and a lumen-like opening substantially through the center of the longitudinal axis of the carrier. The carrier serves as a substrate to provide a physical structure to support the electrodes and conductor lines embedded in a thin polyfluorocarbon film, where said film is heat bonded to the carrier. The insertion of a beading into the lumen-like hole in the carrier allows the surgeon to control the shape of the carrier by pushing or pulling on the beading which is preferentially attached to the apical end of the carrier. Partially circumferential notches (or xe2x80x9cVxe2x80x9d grooves) in the combined film/carrier assembly provide flexibility for insertion of the array into one of the scala of the cochlea and for controlling flexure to conform to the conical helix shape of the scala. In the preferred embodiment, said partially circumferential notches are disposed ad-modiolar (ie. on the inside of the conical helix or spiral shaped array). In an alternate embodiment, said notches are disposed ex-modiolar (ie. on the outside of the conical helix or spiral shaped array). Said notches are preferentially disposed along the longitudinal axis of the film-carrier assembly such that one or more of said notches are disposed between each group of bipolar radial electrodes. In still a further embodiment, said notches are disposed between every second, third or more groups of bipolar radial electrodes.
In two additional embodiments of the invention, the electrode array is comprised of either:
(a) a bio-inert film holding conductor lines and at least two radial bipolar oriented electrodes, or
(b) a bio-inert film, holding conductor lines and at least two radial bipolar oriented electrodes, and a bio-inert carrier, with at least one partially circumferential notch (or xe2x80x9cVxe2x80x9d groove) disposed along the longitudinal axis of the carrier.
Those skilled in the art will appreciate that the film, carrier and beading can be made from, for example, any one of the various polyfluorocarbons, polyethylene, polypropylene polyimide, polyamide or other bio-inert organic polymers.
The combined film-carrier-beading structure (or alternatively the film structure, or the film/carrier structure) containing the film-embedded electrodes and conductors (referred to hereinafter as the electrode array or array) is preferentially shaped into a three-dimensional conical helix, (referred to hereinafter as the conical helix) substantially conforming to the three-dimensional conical helix shape of the scala modiolar wall. The said array has one conical helix configuration to fit the scalae of the right ear and a mirror image conical helix configuration to fit the scalae of the left ear. In this document, the term diameter, when it refers to the conical helix or to the spiral shape of the electrode array, describes the decreasing diameter of the conical helix or spiral from the basal end to the apical end of said array.
In an alternate embodiment, the array can be shaped into a two-dimensional spiral. In a further embodiment, the array can be left substantially straight (or non pre-shaped) for insertion into a scala. Formation of the array into a conical helix or spiral shape is accomplished by heating said array to near the melting or softening point, and holding said array at approximately such temperature for sufficient time to allow the material to substantially retain said conical helix or spiral shape upon cooling. The array is thus xe2x80x9cnormalizedxe2x80x9d (ie. the permanent change from one shape to another shape via heating and subsequent cooling back to ambient temperature) substantially retaining said conical helix or spiral shape at room or body temperature. An additional embodiment of the invention is to create a conical helix or spiral shape that is up to 50% smaller in diameter than the diameter of the conical helix shape of the human scalae modiolar wall so as to position the array to xe2x80x9chugxe2x80x9d the modiolar wall upon insertion. In another embodiment, the beading is xe2x80x9cnormalizedxe2x80x9d while the film-carrier assembly is not, such that the beading serves to induce curvature of said film-carrier assembly upon insertion into the cochlear scala.
For the preferred embodiment, the beading is made of the polyfluorocarbon FEP, a bio-inert polymer. For the preferred embodiment, the beading and the hole both have substantially rectangular cross-sectional shapes, where the longer dimension of the rectangle is parallel to the flat portion of the cross-section of the carrier. The rectangular shaped polymer beading is designed to slide smoothly within the carrier hole, where the orientation of the rectangular beading now serves to assist with preferential curving of the electrodes on the carrier towards the scala modiolar wall. In alternate embodiments, the cross-sectional dimension of the beading may be substantially smaller than the cross-sectional dimension of the hole, wherein said beading slides smoothly within the hole in said carrier.
While the preferential longitudinal shape of the carrier is tapered, being larger in cross-section at the basal end and smaller at the apical end, in an alternate embodiment, said carrier has a longitudinal shape that is not tapered, having substantially the same cross-sectional dimension along its entire length. The cross-section of the carrier having ad-modiolar notches has one rounded surface and one substantially flat surface in the preferred embodiment. Another embodiment, includes a substantially oval cross-section for the carrier with ex-modiolar notches. Said carrier, in further embodiments, can have a cross-section that is round, oval, square, rectangular, triangular, or substantially any other shape.
In the preferred embodiment, the longitudinal hole in the carrier extends completely through the length of said carrier from the basal end to the apical end. In another embodiment, said hole extends from the basal end only partially through the length of said carrier.
For the preferred embodiment containing ad-modiolar disposed partially circumferential notches, pushing on the beading also serves to straighten not only the apical end of the carrier but also the entire body of the carrier. For an alternate embodiment, containing ex-modiolar disposed partially circumferential notches, pulling on the beading also serves to straighten not only the apical end of the carrier but also the entire body of the carrier.
For the preferred embodiment, the beading cross-section is substantially the same along the longitudinal length. In an alternate embodiment, the beading cross-section is tapered, being larger at the basal end and smaller at the apical end. In a further embodiment, the beading can alternatively be comprised of a biocompatible wire or memory shape wire (see for example, U.S. Pat. No. 5,800,500 by F. Spelman, et. al.), either bare or inside a flexible tube, where such beading is used to shape the electrode array for surgical insertion and or assist in positioning the electrode array closer to the modiolus. The beading may, or may not, be removed from the array after surgical insertion of the array.
The apical end of the beading is preferentially attached to the carrier apical end, with the external part of the carrier apical end rounded to minimize trauma during insertion, and the carrier basal end attached to an insertion tool. In the preferred embodiment, for the array containing ad-modiolar notches, said tool allows the surgeon to manually move the beading gently inwards (towards the apical end) during insertion into the scala, such movement acting to straighten the highly flexible array tip, and the entire array, preventing said tip from curling over onto itself or puncturing the basilar membrane. In an alternate embodiment, for the array containing ex-modiolar notches, said tool allows the surgeon to manually move the beading gently outwards (away from the apical end) during insertion into the scala, such movement acting to straighten the highly flexible array tip, and the entire array, preventing said tip from curling over onto itself or puncturing the basilar membrane. In a further embodiment, the beading is not attached to the array apical end, and can be removed after insertion, or simply left in the carrier hole.
The use of said insertion tool by the surgeon facilitates array insertion into the scala by conventional entry via elevation of a postauricular skin flap and exposure of the mastoid cortex, or via the auditory canal
Upon insertion of the apical end of the electrode array to substantially the location of the cochlear helicotrema, the beading at the basal end may be pulled by the surgeon to snug the array to the modiolar wall, after which the insertion tool is detached from the array. In one embodiment, said beading is then cut near the point of array entry into the bony cochlea. A further embodiment of the invention includes means to tighten and anchor said beading at the basal end post-insertion so as to more tightly and permanently position the array against the modiolar wall. In a further embodiment, the beading is simply removed by the surgeon. Note that the beading serves two key functions:
(a) to allow the conical helix or spiral shaped electrode array to be substantially straightened for convenient surgical insertion of the array into a scala, or into an insertion tool for subsequent insertion into a scala, and
(b) to allow the diameter of the conical helix, spiral or straight shaped array to be made smaller post-insertion (thereby more closely hugging the modiolar wall) by pulling (or pushing) on the beading.
In the preferred embodiment, the metal used for both electrodes and conductors is platinum. Other bio-inert metals such as iridium, gold, tantalum, rhodium, rhenium or alloys thereof, and one or more coatings of one metal over the other, may also be used. The metal conductors and radially oriented multi-bipolar metal electrodes can be formed using a variety of standard technologies.
The metal electrodes and metal conductors can be fabricated simultaneously, and subsequently sandwiched between two thin layers of film. Said film layers can be subsequently bonded together, where said films are comprised of a bio-inert organic polymer, and the electrode surfaces subsequently opened to expose the electrode metal surface. The shape of the exposed metal electrode surfaces, in the preferred embodiment, is square. In other embodiments, the shape of said exposed electrode surfaces can be rectangular, round, oval, triangular or substantially any other configuration. The planar film layer is then bonded to the carrier, substantially encapsulating said carrier to create the array structure, with, in one embodiment, the ad-modiolar partially circumferential notches in the array being pre-fonned during molding of the carrier, with one or more notches positioned between groups of multi-bipolar oriented electrodes. xe2x80x9cCut-outsxe2x80x9d are preferentially disposed in the film between the groups of electrodes, wherein said xe2x80x9ccut-outsxe2x80x9d match the notches in the carrier when the film is bonded to the carrier. In another embodiment, said notches in the carrier are mechanically cut out. Said notches allow the surgeon to rotationally orient the array in a scala, preferably the scala tympani, such that the multi-bipolar electrodes are oriented to face towards the modiolus and or spiral lamina.
In an alternative embodiment of the invention, wherein the electrode array is composed solely of a film containing electrodes and conductor lines, said xe2x80x9ccut-outsxe2x80x9d are preferentially disposed in the film between the groups of electrodes. Upon forming the film into a tube-like conformation, said xe2x80x9ccut-outsxe2x80x9d allow normalization of the array into a conical helix or spiral, and facilitate rotational stability of the array within the scala upon insertion. In this embodiment, the array can be positioned in the scala such that the xe2x80x9ccut-outsxe2x80x9d are oriented either admodiolar or exmodiolar.
In still another embodiment of the invention, the film/carrier assembly forms the complete electrode array. In this embodiment, there is no hole down the longitudinal axis of the carrier and there is no beading. Partially circumferential notches are preferentially disposed ad-modiolar and the array is preferentially normalized into a conical helix (or spiral) shape to facilitate positioning of said array to hug the modiolar wall of the scala. An alternate embodiment involves the disposition of the partially circumferential notches in an ex-modiolar location along the length of the carrier. Without the beading, the surgeon has a narrow span of control over the array during insertion, however, the conical helix (or spiral) shape of the array assures that the electrodes are positioned in close proximity to the spiral lamina and the spiral ganglion cells.
To allow for both movement and future head growth in implanted infants and children, the conductor lines between the electrode array and the electronics package contain a series of pleats to allow the device to xe2x80x9cstretchxe2x80x9d so as to prevent breakage of said conductor lines or spontaneous explantation of the array from the cochlear scala(e).
An alternate embodiment includes array insertion into the scala vestibuli, where the electrodes are oriented to face towards the modiolus. A yet further embodiment is the insertion of arrays into both the scala tympani and the scala vestibuli, such that electric fields between electrode pairs within the same scala, or between scalae, can be configured to have electric field lines substantially parallel to nerve fibers in the spiral lamina and or the spiral ganglia in the modiolus.
For safety reasons, it is essential that the array be surgically removable in case of failure, infection or for any other reason. Since the inventive features of the array do not contain any substantial protruding elements, and since the partially circumferential notches allow convenient flexure of the overall array, explantation of said array is feasible.
Other aspects of the invention will be appreciated by reference to the detailed description of the invention and to the claims.